An overwhelming number of U.S. presidents and Nobel Laureates are first borns, not to mention 21 of the original 23 NASA Astronauts. What could possibly account for this trend?
In 2007, Norwegian epidemiologists Petter Kristensen and Tor Bjerkedal published work showing a small but reliable negative relationship between IQ and birth order for 250,000 families. As it turns out, the more siblings one has, the lower one’s IQ . What could possibly explain this relationship? We have four theories:
1. Equity heuristic
Family size does play a role. Parents want to invest the same amount of time and resources in each child, but there is one problem – parental attention needs to be shared when there is more than one child. With two, the attention is halved. With three – there is even less attention to go around. The fact that astronauts are disproportionately firstborns, for example, could just indicate that they come from smaller families – not that firstborns have any particular astronautic qualities.
The reason for this is that more children means parental resources (namely time, attention and money) become spread more thinly across the siblings. Perhaps even more telling is that family size is correlated with many social factors including ethnicity, education and wealth. For example, wealthier, more educated individuals typically have fewer children. If astronauts are more likely to come from well-educated, financially comfortable parents, then they are more likely to be part of a smaller family and thus more likely to be first borns.
2. No one to teach
Time and time again, research in education has shown that the best way to learn something is to teach it. Older siblings have a disproportionately larger opportunity to teach their younger siblings life lessons, while the babies of the family have fewer chances to share their wisdom. It has been proposed that the experience of teaching younger siblings third grade maths reinforces older children’s mathematical learnings, helping them to get better grades in school.
3. Confluence theory
The idea is that children grow up to be smarter if their home environment is an ‘intellectual culture’. If you treat a child like a small human, their maturity will grow accordingly. But as parents become more experienced in raising children, it will become less likely that they treat their children as mini adults. Consequently, younger children often enter a more ‘diluted’ intellectual environment than the siblings born before them.
4. Evolutionary argument
Each child wants to receive attention, but first they have to claim their niche. One sibling wants to become an actor, while another might aspire to be a star athlete, while another gets straight A’s . The privilege of being born first is that the oldest gets to choose which niche suits them best, before the competition arises. And it seems that the eldest children tend to choose intellectual achievements. Researchers from the University of Essex in the UK found first borns are 16% more likely to excel academically that their younger siblings . This is especially true for girls: eldest daughters are an additional four percent more likely to go on to higher education that families with first born sons. These results held up in the study, regardless of parents’ education and professional achievements.
For decades we thought that birth order had an impact on intelligence, but it seems that it is more a matter of the resources provided to each child. The less children you have, the more resources they will receive – primarily time, money, and attention – and the more intelligent they will grow up to be. There is no doubt that there are some benefits to being the first born child, but it does not guarantee you a place at NASA!
 Kristensen, P. & Bjerkedal, T (2007) Explaining the relation between birth order and intelligence. Science, 316, pg. 1717. DOI: 10.1126/science.1141493
 Sullway, F (1996) Born to rebel. New York, Pantheon Books.
 Bu, F. (2014) Sibling configurations, educational aspiration and attainment. Institute for social and economic research, ISER Working Paper.